To properly debug Linux a serial console is needed. Tegra30 has 5 hardware UART ports. They are called UART-A/B/C/D/E
UART-A is on the ULPI data pins on the SoC (https://github.com/u-boot/u-boot/blob/master/arch/arm/mach-tegra/tegra30/pinmux.c#L25)
Below you can see where you can find the UARTA TX, RX pad.
To use UART you need i.e. a usb-uart adapter, a raspberry pi, arduino, etc... A "silicon labs cp210x usb to uart bridge" was used in testing. This is a 3V3 chip which is 5V tolerant. Receiving a 1.8V signal works fine. But sending 3.3V to a 1.8V input is dangerous.
We took an oscilloscope and checked that there isn't a pin which provides more than 5V (max of uart adapter).
After this we probed every pin with a usb uart adapter and minicom. After a few seconds we found the corresponding TX-pad.
dmesg -Tw show that sRT is booted in APX mode. Also shows if it gets disconnected.
Bottom left: Fusee Gelee launcher with payload and ProductID (see dmesg)
Top right: Minicom output
Bottom right: Fusee gelee payload source code
At this time we got uboot booting. We got output from uboot over UART-A-TX so we only had to bring an TX signal from pc to the RX pad on surfaceRT.
One educated guess later we found RX next to TX.
Uboot told us that
1 isn't a valid command. after this we tried
help and and got a solid answer